Display device and electronic device using the same

ABSTRACT

A display device with a compensation circuit that applies a fixed potential constantly to a gate electrode of a driving transistor for a certain period is provided. Specifically, each difference voltage value between an anode and a cathode of the light emitting element is utilized in the case where the light emitting element emits light and emits no light. In a case where the light emitting element emits light, a potential of the gate electrode of the driving transistor is to be held; and in a case where the light emitting element emits no light, a potential that certainly turns off the gate electrode of the driving transistor is kept on applying to the gate electrode of the driving transistor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device using a light emittingelement. Specifically, the present invention relates to an active-matrixtype display device including a plurality of pixels that are arranged inmatrix where each of the plurality pixels includes a light emittingelement. Further, the present invention also relates to an electronicdevice using the display device.

2. Description of the Related Art

In recent years, a display device using a self-light emitting elementhas been actively researched and developed. Especially, a display deviceusing a light emitting element typified as an electro luminescence (EL)element has been actively researched and developed for practicalapplication of use of a display.

In the case of displaying a multi-gray scale image by a display deviceusing a light emitting element, a driving method of an analog drivingmethod (an analog gray scale method) or a digital driving method (adigital gray scale method) is employed. The analog driving method is amethod in which current magnitude flowing in a light emitting element iscontinuously controlled to obtain a gray scale. The digital drivingmethod is a method in which a light emitting element is operated byusing only two states of an ON state (where the luminance isapproximately 100% and the light emitting element emits light) and anOFF state (where the luminance is approximately 0% and the lightemitting element emits no light).

As the digital driving method without modification, the light emittingelement can display only two gray scales because the light emittingelement is operated by using only two states of the ON state and the OFFstate. Therefore, a method, which is combined with a driving methoddisplaying a multi-gray scale such as an area gray scale method and atime gray scale method, is employed. The area gray scale method is amethod in which a gray scale display is performed depending on the sizeof a light emitting area of a sub-pixel by providing the sub-pixel in apixel (for example, see Patent Document 1). Further, the time gray scalemethod is a method in which a gray scale is displayed by controlling alight-emitting period and frequency of light emission of a pixel (forexample, see Patent Document 2 and Patent Document 3).

[Patent Document 1] Japanese Patent Application Laid-Open No. H11-73158

[Patent Document 2] Japanese Patent Application Laid-Open No. 2001-5426

[Patent Document 3] Japanese Patent Application Laid-Open No.2001-343933

A configuration example of a pixel in a display device employing thedigital driving method and operation thereof will be described below.Note that it is difficult to define a source electrode and a drainelectrode of a thin film transistor over a substrate having an insulatedsurface due to a structure thereof. Accordingly, hereinafter, one of asource electrode and a drain electrode is represented as a firstelectrode, and then, the other of the source electrode and the drainelectrode is represented as a second electrode except for a case where adefinition of a source electrode and a drain electrode is particularlyneeded. In general, a lower potential side is a source electrode and ahigher potential side is a drain electrode in an N-channel transistor,whereas a higher potential side is a source electrode and a lowerpotential side is a drain electrode in a P-channel transistor.

A pixel 210 includes a writing transistor 203, a driving transistor 205,and a light emitting element 206 (see FIG. 12). In the writingtransistor 203, a gate electrode, a first electrode, and a secondelectrode are respectively connected to a scanning line 202, a signalline 200, and a gate electrode of the driving transistor 205. In thedriving transistor 205, a first electrode and a second electrode arerespectively connected to a power source line 201, and a first electrodeof the light emitting element 206. A second electrode of the lightemitting element 206 is connected to a power source 207. In the lightemitting element 206, a structure in which the first electrode is ananode and the second electrode is a cathode, or a structure in which theanode and the cathode are reversed may be employed. In this case, acurrent direction is changed; therefore, each potential of the powersource line 201 and the power source 207 are appropriately determined.

A capacitor 204 is provided to hold voltage between the gate electrodeand the source electrode of the driving transistor 205 (hereinafter,referred to as Vgs). The capacitor 204 may be provided between the gateelectrode of the driving transistor 205 and the power source line 201;or between the gate electrode of the driving transistor 205 and a wiringthat is held at a fixed potential. Alternatively, the capacitor 204 forholding the voltage between the gate electrode and the source electrodeof the driving transistor 205 is not provided, and instead, a parasiticcapacitance that parasitizes between the gate electrode and the sourceelectrode of the driving transistor 205 may be used in order to hold theVgs of the driving transistor 205.

The power source line 201 and the power source 207 are held at eachpredetermined potential, which have a potential difference in eachother. The power source line 201 is held at a fixed potential. Thesecond electrode of the light emitting element 206 that is connected tothe power source 207 is held at a fixed potential.

When the writing transistor 203 is turned on by a signal inputtedthrough the scanning line 202, a video signal is inputted into the gateelectrode of the driving transistor 205 through the signal line 200. Apotential difference between the video signal and the power source line201 is to be the Vgs of the driving transistor 205. When the drivingtransistor 205 is turned on by value of the Vgs, current is supplied tothe light emitting element 206, and the light emitting element 206 emitslight. On the other hand, when the driving transistor 205 is turned offby value of the Vgs, current is not supplied to the light emittingelement 206, and the light emitting element 206 emits no light. The Vgsof the driving transistor 205 is held by the capacitor 204 for a certainperiod until a next video signal is inputted into the pixel 210.

SUMMARY OF THE INVENTION

Light emission and non-light emission of the light emitting element 206is determined depending on turning on and off of the driving transistor205. Accordingly, it is required that the Vgs of the driving transistor205 is a potential by which the driving transistor 205 is certainlyturned on and off. Further, it is required that the Vgs of the drivingtransistor 205 is held so as not to vary for a certain period.

The capacitor 204 is provided to hold the Vgs of the driving transistor205. However, when miniaturization of the pixel 210 with high definitionproceeds, it has been difficult to secure an area for forming acapacitor that holds a sufficient amount of charge. There has been acase where the Vgs of the driving transistor 205 is held by usingparasitic capacitance of the driving transistor 205 because the area forproviding the capacitor 204 cannot be secured depending on the degree ofminiaturization of the pixel 210.

When capacitance value of the capacitor 204 is insufficient or a leakagecurrent is generated in the writing transistor 203, there has been acase where a potential of the gate electrode of the driving transistor205 varies gradually. In particular, when the pixel 210 displays blackcolor, there has been a case where the driving transistor 205 is notcompletely turned off by a little variation in a potential of the gateelectrode of the driving transistor 205, and a small amount of currentflows in the light emitting element 206 to generate slight lightemission (hereinafter, referred to as black float) in the light emittingelement 206. The black float is a display defect, which is easilyrecognized, and has been a big problem.

In the view of the above condition, it is an object of the presentinvention to provide a display device and an electronic device, whichcan control light emission and non-light emission of a light emittingelement more precisely. Further, it is also an object of the presentinvention to provide a display device and an electronic device, whichcan make a driving transistor turn on and off certainly in order tocontrol light emission and non-light emission of a light emittingelement more precisely.

The present invention provides a display device with a compensationcircuit that applies a fixed potential constantly to a gate electrode ofa driving transistor for a certain period. By providing the compensationcircuit, variation in a potential of the gate electrode of the drivingtransistor can be prevented. Accordingly, in the case of black displayparticularly, generation of black float that is easily to be recognizedas a display defect is suppressed, and light emission and non-lightemission of the light emitting element are controlled more precisely.

Specifically, each voltage value between an anode and a cathode of thelight emitting element is different in the case where the light emittingelement emits light and emits no light. By utilizing the voltage valueat this time, the present invention provides a display device asfollows. In the display device, in the case where the light emittingelement emits light, a potential of the gate electrode of the drivingtransistor is to be held; and in the case where the light emittingelement emits no light, a potential that certainly turns off the drivingtransistor can be applied to the gate electrode of the drivingtransistor.

A display device of the present invention includes a plurality ofpixels, each of which includes a first transistor, a second transistor(corresponding to a driving transistor), a light emitting element, and acircuit that controls conduction between a gate electrode of the secondtransistor and a power source line (corresponding to a compensationcircuit).

In a display device having the above structure, in the first transistor,a gate electrode; one of a source electrode and a drain electrode; andthe other of the source electrode and the drain electrode areelectrically connected to a scanning line; a signal line; and the gateelectrode of the second transistor, respectively. In the secondtransistor, one of a source electrode and a drain electrode; and theother of the source electrode and the drain electrode are electricallyconnected to the power source line; and a first electrode of the lightemitting element, respectively. In the light emitting element, a secondelectrode is held at a fixed potential. The circuit is to be in aconduction state or a non-conduction state depending on a potential ofthe first electrode of the light emitting element and a potential of thescanning line. A potential of the power source line is held to bedifferent from the potential of the second electrode of the lightemitting element.

A display device of the present invention includes a plurality ofpixels, each of which includes a first transistor, a second transistor(corresponding to a driving transistor), a light emitting element, acircuit that controls conduction between a gate electrode of the secondtransistor and a power source line (corresponding to a compensationcircuit), and a controlling element that controls conduction between thegate electrode of the second transistor and a second scanning line.

In a display device having the above structure, in the first transistor,a gate electrode; one of a source electrode and a drain electrode; andthe other of the source electrode and the drain electrode areelectrically connected to a first scanning line; a signal line; and thegate electrode of the second transistor, respectively. In the secondtransistor, one of a source electrode and a drain electrode; and theother of the source electrode and the drain electrode are electricallyconnected to a power source line; and a first electrode of the lightemitting element, respectively. In the light emitting element, a secondelectrode is held at a fixed potential. The circuit is to be in aconduction state or a non-conduction state depending on a potential ofthe first electrode of the light emitting element and a potential of thefirst scanning line. The controlling element is to be in a conductionstate or a non-conduction state based on a potential of a secondscanning line. A potential of the power source line and the potential ofthe second scanning line are held to turn off the second transistor.

Further, in a display device having the above structure, in the firsttransistor, a gate electrode; one of a source electrode and a drainelectrode; and the other of the source electrode and the drain electrodeare electrically connected to a first scanning line; a signal line; andthe gate electrode of the second transistor, respectively. In the secondtransistor, one of a source electrode and a drain electrode; and theother of the source electrode and the drain electrode are electricallyconnected to a power source line; and a first electrode of the lightemitting element, respectively. In the light emitting element, a secondelectrode is held at a fixed potential. The circuit is to be in aconduction state or a non-conduction state depending on a potential ofthe first electrode of the light emitting element, a potential of thefirst scanning line, and a potential of a second scanning line. Thecontrolling element is to be in a conduction state or a non-conductionstate based on a potential of the second scanning line. A potential ofthe power source line and a potential of the second scanning line areheld to turn off the second transistor.

A display device of the present invention includes a plurality ofpixels, each of which includes a first transistor, a second transistor(corresponding to a driving transistor), a light emitting element, acircuit that controls conduction between a gate electrode of the secondtransistor and a power source line (corresponding to a compensationcircuit), and a controlling element that controls conduction between oneof a source electrode and a drain electrode of the second transistor andthe power source line.

In a display device having the above structure, in the first transistor,a gate electrode; one of a source electrode and a drain electrode; andthe other of the source electrode and the drain electrode areelectrically connected to a first scanning line; a signal line; and thegate electrode of the second transistor, respectively. In the secondtransistor, one of the source electrode and the drain electrode; and theother of the source electrode and the drain electrode are electricallyconnected to the power source line through the controlling element; anda first electrode of the light emitting element, respectively. In thelight emitting element, a second electrode is held at a fixed potential.The circuit is to be in a conduction state or a non-conduction statedepending on a potential of the first electrode of the light emittingelement and a potential of the first scanning line. The controllingelement is to be in a conduction state or a non-conduction statedepending on a potential of a second scanning line. A potential of thepower source line is held to be different from a potential of the secondelectrode of the light emitting element.

A display device of the present invention includes a plurality ofpixels, each of which includes a first transistor, a second transistor,a light emitting element, a third transistor, and a fourth transistor.In the first transistor, a gate electrode; one of a source electrode anda drain electrode; and the other of the source electrode and the drainelectrode are electrically connected to a scanning line; a signal line;and a gate electrode of the second transistor and one of a sourceelectrode and a drain electrode of the third transistor, respectively.In the second transistor, one of a source electrode and a drainelectrode; and the other of the source electrode and the drain electrodeare electrically connected to a power source line; and a first electrodeof the light emitting element and a gate electrode of the thirdtransistor. In the light emitting element, a second electrode is held ata fixed potential. In the third transistor, the other of the sourceelectrode and the drain electrode is electrically connected to one of asource electrode and a drain electrode of the fourth transistor. In thefourth transistor, a gate electrode; and the other of the sourceelectrode and the drain electrode are electrically connected to thescanning line; and the power source line, respectively. A potential ofthe power source line is held to turn off the second transistor.

A display device of the present invention includes a plurality ofpixels, each of which includes a first transistor, a second transistor,a light emitting element, a third transistor, a fourth transistor, and acontrolling element. In the first transistor, a gate electrode; one of asource electrode and a drain electrode; and the other of the sourceelectrode and the drain electrode are electrically connected to a firstscanning line; a signal line; and a gate electrode of the secondtransistor, one of a source electrode and a drain electrode of the thirdtransistor, and one of terminals of the controlling element,respectively. In the second transistor, one of a source electrode and adrain electrode; and the other of the source electrode and the drainelectrode are electrically connected to a power source line; and a firstelectrode of the light emitting element and a gate electrode of thethird transistor, respectively. In the light emitting element, a secondelectrode is held at a fixed potential. In the third transistor, theother of the source electrode and the drain electrode is electricallyconnected to one of a source electrode and a drain electrode of thefourth transistor. In the fourth transistor, a gate electrode; and theother of the source electrode and the drain electrode are electricallyconnected to the first scanning line; and the power source line,respectively. In the controlling element, the other terminal isconnected to a second scanning line. A potential of the power sourceline and a potential of the second scanning line are held to turn offthe second transistor.

A display device of the present invention includes a plurality ofpixels, each of which includes a first transistor, a second transistor,a light emitting element, a third transistor, a fourth transistor, afifth transistor, and a controlling element. In the first transistor, agate electrode; one of a source electrode and a drain electrode; and theother of the source electrode and the drain electrode are electricallyconnected to a first scanning line; a signal line; and a gate electrodeof the second transistor and one of a source electrode and a drainelectrode of the third transistor, respectively. In the secondtransistor, one of a source electrode and a drain electrode; and theother of the source electrode and the drain electrode are electricallyconnected to a power source line; and a first electrode of the lightemitting element, a gate electrode of the third transistor, and one ofterminals of the controlling element, respectively. In the lightemitting element, a second electrode is held at a fixed potential. Inthe third transistor, the other of the source electrode and the drainelectrode is electrically connected to one of a source electrode and adrain electrode of the fourth electrode. In the fourth transistor, agate electrode; and the other of the source electrode and the drainelectrode are electrically connected to the first scanning line; and oneof a source electrode and a drain electrode of the fifth transistor,respectively. In the fifth transistor, the other of the source electrodeand the drain electrode is electrically connected to the pour sourceline. In the controlling element, the other terminal is connected to asecond scanning line. A potential of the power source line and apotential of the second scanning line are held to turn off the secondtransistor.

In a display device having the above structure, the controlling elementis a diode. Further, in a display device having the above structure, thecontrolling element is a sixth transistor. One of terminals of thecontrolling element is a gate electrode and a drain electrode of thesixth transistor, and the other terminal of the controlling element is asource electrode of the sixth transistor. In addition, in a displaydevice having the above structure, the first transistor, the secondtransistor, the light emitting element, the third transistor, the fourthtransistor, the fifth transistor, and the sixth transistor are providedover a substrate having a same insulated surface. When the sixthtransistor is used as a controlling element, first to fifth transistorsincluded in a pixel and the sixth transistor can be manufactured in asame manufacturing process. Accordingly, by using the sixth transistoras a controlling element, a manufacturing process is not required to beadded; therefore, a display device can be manufactured easily.

A display device of the present invention includes a plurality ofpixels, each of which includes a first transistor, a second transistor,a light emitting element, a third transistor, a fourth transistor, and afifth transistor. In the first transistor, a gate electrode; one of asource electrode and a drain electrode; and the other of the sourceelectrode and the drain electrode are electrically connected to a firstscanning line; a signal line; and a gate electrode of the secondtransistor and one of a source electrode and a drain electrode of thethird transistor, respectively. In the second transistor, one of asource electrode and a drain electrode; and the other of the sourceelectrode and the drain electrode are electrically connected to one of asource electrode and a drain electrode of the fifth transistor; and afirst electrode of the light emitting element and a gate electrode ofthe third transistor, respectively. In the light emitting element, asecond electrode is held at a fixed potential. In the third transistor,the other of the source electrode and the drain electrode iselectrically connected to one of a source electrode and a drainelectrode of the fourth transistor. In the fourth transistor, a gateelectrode; and the other of the source electrode and the drain electrodeare electrically connected to the first scanning line; and a powersource line, respectively. In the fifth transistor, a gate electrode;and the other of the source electrode and the drain electrode areelectrically connected to a second scanning line; and the power sourceline, respectively.

In a display device having the above structure, the controlling elementis the fifth transistor. In the fifth transistor, a gate electrode; oneof a source electrode and a drain electrode; and the other of the sourceelectrode and the drain electrode are electrically connected to thesecond scanning line; the power source line; and one of a sourceelectrode and a drain electrode of the second transistor, respectively.

In a display device having the above structure, the third transistor maybe a transistor having a plurality of gate electrodes that are connectedto each other. In other words, the third transistor may be replaced witha transistor having a plurality of gate electrodes that are connected toeach other (also called a transistor of a multi-gate structure). Byusing the transistor having a plurality of gate electrodes that areconnected to each other, a leakage current can be reduced.

In a display device having the above structure, polarity (also referredto as a conductivity type) of the first transistor and polarity of thefourth transistor are different each other.

In a display device of the present invention, a driving method, whichcan realize a multi-gray scale display with high precision by using adigital driving method and a digital time gray scale method, can beapplied.

Further, an electronic device (a portable information terminal; a camerasuch as a digital camera, a video camera, and a digital video camera; acellular phone unit; a portable television set; a computer such as anotebook computer; or the like) of the present invention uses a displaydevice having any one of the above structures.

In accordance with the present invention, light emission and non-lightemission of a light emitting element can be controlled more precisely bysuppressing variation in a potential of a gate electrode of a drivingtransistor. Accordingly, generation of a display defect can besuppressed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a configuration example of a pixel in adisplay device of the present invention;

FIG. 2 is a diagram showing a timing chart;

FIGS. 3A and 3B are diagrams showing a configuration example of a pixelin a display device of the present invention;

FIGS. 4A and 4B are diagrams showing a configuration example of a pixelin a display device of the present invention;

FIG. 5 is a diagram showing a configuration example of a pixel in adisplay device of the present invention;

FIG. 6 is a diagram showing a configuration example of a pixel in adisplay device of the present invention;

FIG. 7 is a diagram describing a display device of the presentinvention;

FIGS. 8A and 8B are views describing a display device of the presentinvention;

FIG. 9 is a view showing an electronic device using a display device ofthe present invention;

FIGS. 10A to 10E are views showing electronic devices using a displaydevice of the present invention;

FIG. 11 is a view showing a layout of a pixel; and

FIG. 12 is a diagram showing a configuration example of a pixel in adisplay device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiment Modes of the present invention will be described in detailswith reference to the accompanying drawings. However, the presentinvention is not limited to the following description, and it is to beeasily understood that various changes and modifications are possible bythose skilled in the art, unless such changes and modifications departfrom the content and the scope of the invention. Therefore, the presentinvention is not construed as being limited to the description of thefollowing embodiment modes. It is to be noted that the same portion isdenoted by the same reference numeral in different drawings in thefollowing description.

Embodiment Mode 1

A configuration of a pixel in a display device of the present inventionwill be described with reference to FIG. 1.

A pixel 110 includes a writing transistor 103, a driving transistor 106,a light emitting element 107, and a circuit that controls conductionbetween a gate electrode of the driving transistor 106 and a powersource line 101 (hereinafter, called a compensation circuit 109). Thecompensation circuit 109 includes a transistor 104 and a transistor 105that are connected in series.

Further, the pixel 110 may include a capacitor that holds a potential ofthe gate electrode of the driving transistor 106. A first electrode ofthe capacitor is connected to the gate electrode of the drivingtransistor 106, and a second electrode of the capacitor is connected toa source electrode of the driving transistor 106 or a power source linethat is held at a fixed potential. A case where the potential of thegate electrode of the driving transistor 106 is held by capacitance(parasitic capacitance) of the driving transistor 106 is describedbelow.

In the writing transistor 103, a gate electrode; a first electrode (oneof a source electrode and a drain electrode); and a second electrode(the other of the source electrode and the drain electrode) areconnected to a scanning line 102; a signal line 100; and the gateelectrode of the driving transistor 106 and a first electrode of thetransistor 104, respectively. In the driving transistor 106, a firstelectrode (one of a source electrode and a drain electrode); and asecond electrode (the other of the source electrode and the drainelectrode) are connected to a first electrode of the light emittingelement 107 and a gate electrode of the transistor 104; and the powersource line 101, respectively. In the light emitting element 107, asecond electrode is connected to a terminal 108 that is connected to apower source, and is held at a fixed potential.

In the transistor 104, the gate electrode; a first electrode (one of asource electrode and a drain electrode); and a second electrode (theother of the source electrode and the drain electrode) are connected tothe first electrode of the light emitting element 107 and the firstelectrode of the driving transistor 106; the gate electrode of thedriving transistor 106; and a first electrode of the transistor 105,respectively. In the transistor 105, a gate electrode; a first electrode(one of a source electrode and a drain electrode); and a secondelectrode (the other of the source electrode and the drain electrode)are connected to the scanning line 102; the second electrode of thetransistor 104; and the power source line 101, respectively.

The light emitting element 107 may have a structure in which the firstelectrode is an anode and the second electrode is a cathode, or astructure in which the anode and the cathode are reversed. In that case,a current direction is changed; therefore, a potential of the powersource 101 and a potential of the power source that is connected to theterminal 108 are appropriately determined.

Both the writing transistor 103 and the transistor 105 are connected tothe scanning line 102. Polarity of the writing transistor 103 andpolarity of the transistor 105 are different from each other. When thewriting transistor 103 is turned on, the transistor 105 is turned off.Alternatively, when the writing transistor 103 is turned off, thetransistor 105 is turned on.

If polarity of the writing transistor 103 and polarity of the transistor105 are the same, a scanning line corresponding to each of the writingtransistor 103 and the transistor 105 is provided.

Further, the transistor 104 has polarity, which is turned off when apotential of the power source connected to the terminal 108 istransmitted.

Next, operation of the pixel 110 will be described with the use of atiming chart (see FIG. 2). Hereinafter, the writing transistor 103 is tobe an N-channel transistor, and each of the driving transistor 106, thetransistor 104, and the transistor 105 is to be a P-channel transistoras an example. Further, as a potential of the power source line 101 isto be a standard, and each potential of the signal line 100 and thescanning line 102 is determined. In the signal line 100, a potential ofa H level (high potential) side of a video signal is higher than that ofthe power source line 101, and a potential of a L level (low potential)side is a potential that certainly turns on the driving transistor 106(for example, 0V). In the scanning line 102, a potential of a H level(high potential) side is higher than that of the signal line 100, whichcertainly turns on the writing transistor 103; and a potential of a Llevel (low potential) side is lower than that of the signal line 100,which certainly turns off the writing transistor 103. Each potential ofthe power source line 101 and a second electrode of the light emittingelement 107 is held at a fixed potential.

It is to be noted that polarity of the above transistors and each powersource potential add no limitation to the present invention. It is easyfor those skilled in the art to configure an equivalent circuit byadding modification to polarity of a transistor and each power sourcepotential in accordance with an art concept of the present invention,and it is clear that these structures are included in the presentinvention.

In a term T1, when the scanning line 102 is selected by a gate driverand a potential thereof becomes higher, the writing transistor 103 isturned on. In addition, a video signal is inputted from a source driverinto the pixel 110 through the signal line 100. Then, a potential of thevideo signal is applied to a gate electrode of the driving transistor106 through the writing transistor 103. Turning on and off of thedriving transistor 106 is determined by a potential difference betweenthe video signal and the power source line 101. In a case where thedriving transistor 106 is turned on, the light emitting element 107emits light by supplying current from the power source line 101 to thelight emitting element 107. In a case where the driving transistor 106is turned off, the light emitting element 107 emits no light. In theterm T1, it is supposed that the driving transistor 106 is turned offdue to the video signal inputted into the pixel 110.

When a selection of the scanning line 102 by the gate driver is finishedand the potential of the scanning line 102 becomes lower, the writingtransistor 103 is turned off. Until the scanning line 102 is selectedagain and a next video signal is inputted into the pixel 110, apotential of the gate electrode of the driving transistor 106 is held bythe driving transistor 106.

In a case where the driving transistor 106 is turned off based on avideo signal inputted into the pixel 110 and the light emitting element107 emits no light, a potential of the first electrode of the lightemitting element 107 becomes equal to or more than a potential of thepower source line connected to the terminal 108, and equal to or lessthan a light-emitting starting potential of the light emitting element107 defined as a potential when the light emitting element starts toemit light. Thus, the transistor 104 is turned on. Further, thetransistor 105 is turned on in this case. Therefore, the gate electrodeof the driving transistor 106 is connected to the power source line 101through the transistor 104 and the transistor 105. As a result, apotential of the power source line 101 is transmitted to the gateelectrode of the driving transistor 106, and then, the drivingtransistor 106 is certainly turned off.

In the case where the driving transistor 106 is turned off based on thevideo signal inputted into the pixel 110 in such a manner, thecompensation circuit 109 is to be in a conduction state, and the gateelectrode of the driving transistor 106 and the power source line 101are made to be in a conduction state. Then, the potential of the powersource line 101 is transmitted to the gate electrode of the drivingtransistor 106 so as to turn off the driving transistor 106 certainly.As a result, variation in the potential of the gate electrode of thedriving transistor 106 can be prevented. It is to be noted that thetransistor 104 included in the compensation circuit 109 is controlledbased on a potential of the first electrode of the light emittingelement 107. In addition, the transistor 105 included in thecompensation circuit 109 is controlled by a potential of the scanningline 102. In such a manner, the present invention has a feature that thecompensation circuit 109 is controlled by utilizing each potential ofthe first electrode of the light emitting element 107 and the scanningline 102 without providing an exclusive circuit for controlling thecompensation circuit 109.

In a term T2, when the scanning line 102 is selected by the gate driverand the potential of the scanning line 102 becomes higher, the writingtransistor 103 is turned on. Further, a video signal is inputted fromthe source driver into the pixel 110 through the signal line 100. In theterm T2, it is supposed that the driving transistor 106 is turned on dueto the video signal inputted into the pixel 110.

When the selection of the scanning line 102 by the gate driver isfinished and the potential of the scanning line 102 becomes lower, thewriting transistor 103 is turned off. Then, until a next video signal isinputted into the pixel 110, the potential of the gate electrode of thedriving transistor 106 is held by the driving transistor 106.

In a case where the driving transistor 106 is turned on based on thevideo signal inputted into the pixel 110 and the light emitting element107 emits light, a potential of the first electrode of the lightemitting element 107 becomes the same potential or about the samepotential as that of the power source line 101. Accordingly, thetransistor 104 is turned off. Since the potential of the gate electrodeof the driving transistor 106 is a lower potential, the drivingtransistor 106 is turned on.

When the light emitting element 107 emits light by turning on thedriving transistor 106, its potential may be increased slightly becauseelectric charge that is held in the gate electrode of the drivingtransistor 106 is leaked. However, it is hardly to be a problem becausea current value that is supplied to the light emitting element 107 isdetermined by a potential difference between the power source line 101and the power source connected to the terminal 108 in a case where thedriving transistor 106 operates in a liner region. Further, in a casewhere the driving transistor 106 operates in a saturation region, theluminance is slightly lowered because the current value that is suppliedto the light emitting element 107 is determined by the drivingtransistor 106. However, in a case of a digital driving method,variation in the luminance is seldom recognized because the operation ofthe light emitting element 107 is controlled by using only two states oflight emission and non-light emission.

It is to be noted that the transistor 104 included in the compensationcircuit 109 may be replaced with transistors 600 to 603 that areconnected in series (see FIG. 6). In other words, the transistor 104 maybe replaced with a transistor having a plurality of gate electrodes thatare connected to each other (also called a transistor of a multi-gatestructure). A leakage current can be reduced by using the transistorhaving a plurality of gate electrodes that are connected to each other.

Embodiment Mode 2

In a case of displaying multi-gray scales by a method in which a digitaldriving method and a time gray scale method are combined, a lightemitting period is required to be controlled more minutely. Therefore,operation in which a predetermined pixel is forcibly made to be in anon-lighting state by a control signal is required. In the presentembodiment mode, an example where the present invention is applied to apixel having a function that forcibly makes a non-lighting state will bedescribed with reference to FIG. 3A and FIG. 4A.

A pixel 110 includes a writing transistor 103, a driving transistor 106,a light emitting element 107, a compensation circuit 109 that controlsconduction between a gate electrode of the driving transistor 106 and apower source line 101, and a controlling element 111 that controlsconduction between the gate electrode of the driving transistor 106 anda scanning line 300. The compensation circuit 109 includes a transistor104 and a transistor 105. Further, the controlling element 111 is adiode 301.

It is to be noted that the controlling element 111 may be a transistor400 having a rectification effect, which connects a gate electrode and adrain electrode (it may be denoted as a diode-connected transistor 400as well) (see FIG. 4A). The transistor 400 can be easily formed by thesame process as other transistors included in the pixel 110. It is to benoted that the controlling element 111 can employ any structure as faras it is an element having a rectification effect. Further, as describedabove, in the case of using the transistor 400, a position of the drainelectrode is different depending on whether its polarity is an N-channeltype or a P-channel type; therefore, a connection is appropriatelychanged depending on the polarity.

A connection of a scanning line 102, a signal line 100, the writingtransistor 103, the driving transistor 106, the power source line 101,the light emitting element 107, the transistor 104, and the transistor105 is the same as described in Embodiment Mode 1; therefore, itsdescription is omitted here. In the diode 301, a first electrode (one ofterminals) is connected to the scanning line 300, and a second electrode(the other terminal) is connected to the gate electrode of the drivingtransistor 106 (see FIGS. 3A and 3B). In the diode-connected transistor400, a source electrode; and gate and drain electrodes are connected tothe scanning line 300; and the gate electrode of the driving transistor106, respectively (see FIGS. 4A and 4B). A potential of the scanningline 300 is held to turn off the driving transistor 106.

Next, operation of the pixel 110 is described. It is to be noted thatthe writing transistor 103 is to be an N-channel transistor, and each ofthe driving transistor 106, the transistor 104, the transistor 105, andthe transistor 400 is to be a P-channel transistor as an example.Further, as a potential of the power source line 101 to be a standard,each potential of the signal line 100, the scanning line 102, and thescanning line 300 is determined. In the signal line 100, a potential ofa H level (high potential) side of a video signal is higher than that ofthe power source line 101, and a potential of a L level (low potential)side is a potential that certainly turns on the driving transistor 106(for example, 0V in this case). In the scanning line 102, a potential ofa H level (high potential) side is higher than that of the signal line100 and a potential that certainly turns on the writing transistor 103,and a potential of a L level (low potential) side is lower than that ofthe signal line 100 and a potential that certainly turns off the writingtransistor 103. In the scanning line 300, a potential of a H level (highpotential) side of a signal is higher than that of the signal line 100,and a potential of a L level (low potential) side is lower than that ofthe signal line 100. It is to be noted that polarity of the abovetransistors and each power source potential add no limitation to thepresent invention. It is easy for those skilled in the art to configurean equivalent circuit by adding modification to polarity of a transistorand each power source potential in accordance with an art concept of thepresent invention, and it is clear that theses structures are alsoincluded in the present invention.

Operation of inputting a video signal into the pixel 110 and operationof lighting and non-lighting of the pixel 110 based on the video signalare the same as the description in Embodiment Mode 1, and thus, thedescription thereof is omitted here.

Next, operation in a case where the pixel 110 is forcibly made to be ina non-lighting state is described.

When writing operation of the video signal to the pixel 110 is finished,the gate electrode of the driving transistor 106 is held at a lowpotential in a case of making the light emitting element 107 emit light.Alternatively, in a case of making the light emitting element 107 emitno light, the gate electrode of the driving transistor 106 is held at ahigh potential.

Here, the scanning line 300 is selected by a gate driver at a timing inwhich the pixel 110 is required to be forcibly in a non-lighting state,and then, the scanning line 300 is held at a high potential. Therefore,the controlling element 111 (the diode 301 or the transistor 400) is tobe in a conduction state, and a potential of the gate electrode of thedriving transistor 106 becomes higher. More properly, the potential ofthe gate electrode of the driving transistor 106 becomes a potentialthat is lower than the potential of the high potential side of thescanning line 300 by a value of a threshold voltage of the diode 301. Bythis operation, the driving transistor 106 is turned off, and currentthat is supplied to the light emitting element 107 is blocked. As aresult, a potential of the first electrode of the light emitting element107 is decreased immediately to a potential that is about the same asthat of a power source connected to a terminal 108, and the transistor104 is turned on. In addition, the transistor 105 is turned on. Evenafter the potential of the scanning line 300 becomes lower, a potentialof the power source line 101 is applied to the gate electrode of thedriving transistor 106 through the transistor 104 and the transistor105. Therefore, the driving transistor 106 is held to be turned off. Byabove operation, the pixel 110 that is once forcibly made to be in anon-lighting state keeps on the state of non-lighting unless a nextvideo signal is written.

As described above, in the present invention, the controlling element111 (the diode 301 or the transistor 400) is made to be in a conductionstate at the timing in which the pixel 110 is forcibly made to be in anon-lighting state. Then, the potential of the scanning line 300 istransmitted to the gate electrode of the driving transistor 106, and thedriving transistor 106 is turned off. By transmitting the potential ofthe scanning line 300 in such a manner, the driving transistor 106 isturned off; however, there is a possibility that variation in apotential is caused in the gate electrode of the driving the transistor106 in this situation. Therefore, in the present invention, by makingthe compensation circuit 109 to be in a conduction state, the gateelectrode of the driving transistor 106 and the power source line 101are made to be in a conduction state. Then, the potential of the powersource line 101 is transmitted to the gate electrode of the drivingtransistor 106. Accordingly, variation in a potential of the gateelectrode of the driving transistor 106 can be prevented, and thedriving transistor 106 can be certainly turned off.

The scanning line 300 is selected to be held at a high potential, andthe controlling element 111 (the diode 301 or the transistor 400) ismade to be in a conduction state. Then, after the potential of thescanning line 300 is transmitted to the gate electrode of the drivingtransistor 106 through the controlling element 111 (the diode 301 or thetransistor 400), the driving transistor 106 is turned off and the lightemitting element 107 emits no light. Thereafter, a potential of thefirst electrode of the light emitting element 107 is decreasedimmediately to a potential that is about the same as that of the powersource connected to the terminal 108, and the transistor 104 is turnedon.

On the other hand, since the transistor 105 is turned on, a penetrationpath is caused between the scanning line 300 and the power source line101 through the transistors 104 and 105 and the controlling element 111.In this case, if a potential in a case where the scanning line 300 isheld at a high potential is the same as a potential of the power source101, there is no problem. However, if there is a potential differencebetween the two wirings, current flows.

Thus, by providing a transistor 112 as shown in FIG. 3B and FIG. 4B, astructure in which the transistor 112 is turned off to block a path ofthe penetration when the scanning line 300 is held at a high potentialmay be employed. A position where the transistor 112 is arranged is notparticularly limited as far as the penetration path can be blocked. Forexample, the transistor 112 may be provided between the transistor 104and the transistor 105. Further, the transistor 112 may be providedbetween one terminal of the controlling element 111 and the transistor104. The transistor 112 is a transistor having polarity such that thetransistor is turned off by a signal transmitted from the scanning line300 when the controlling element 111 is in a conduction state by thesignal transmitted form the scanning line 300.

In order to hold the potential of the gate electrode of the drivingtransistor 106 during the period of the lighting of the pixel 110, it ispreferable to make a leakage current of the transistor 104 smaller thanthat of the diode 301 or the transistor 400. Therefore, the transistor104 may be replaced with the transistors 600 to 603 connected in series(see FIG. 6). In other words, the transistor 104 may be replaced with atransistor having a plurality of gate electrodes that are connected toeach other. A leakage current can be further reduced, by using thetransistor having a plurality of gate electrodes that are connected toeach other.

Embodiment Mode 3

An example in which the present invention is applied to a pixel 110having a function that forcibly makes a non-lighting state will bedescribed with reference to FIG. 5.

The pixel 110 includes a writing transistor 103, a driving transistor106, a power source line 101, a light emitting element 107, acompensation circuit 109 that controls conduction between a gateelectrode of the driving transistor 106 and the power source line 101,and a controlling element 111 that controls conduction between a firstelectrode of the driving transistor 106 and the power source line 101.The compensation circuit 109 includes a transistor 104 and a transistor105. The controlling element 111 is a transistor 501.

A connection of a scanning line 102, a signal line 100 to which a videosignal is inputted, the writing transistor 103, the driving transistor106, the power source line 101, the light emitting element 107, thetransistor 104, and the transistor 105 is the same as the description inEmbodiment Mode 1; therefore the description thereof is omitted. Thetransistor 501 is connected between a second electrode (a sourceelectrode or a drain electrode) of the driving transistor 106 and thepower source line 101 in series, and a gate electrode thereof isconnected to a scanning line 500.

Next, operation of the pixel 110 is described. In order to thedescription of the operation clear, as an example, the writingtransistor 103 is to be an N-channel transistor, and each of the drivingtransistor 106, the transistor 104, the transistor 105, and thetransistor 501 is to be a P-channel transistor. Further, as thepotential of the source line 101 to be a standard, each potential of asignal line 100 and the scanning lines 102 and 500 is determined. In thesignal line 100, a potential of a H level (high potential) side of avideo signal is higher than that of the power source line 101, and apotential of a L level (low potential) side is a potential thatcertainly turns on the driving transistor 106 (for example, 0V in thiscase). In the scanning line 102, a potential of a H level (highpotential) side of a signal is higher than that of the signal line 100and a potential that certainly turns on the writing transistor 103, anda potential of a L level (low potential) side is lower than that of thesignal line 100 and a potential that certainly turns off the writingtransistor 103. In the scanning line 500, a potential of a H level (highpotential) side of a signal is higher than that of the power source line101, and a potential of a L level (low potential) side is the same as orlower than that of the signal line 100. It is to be noted that polarityof the above transistors and each power source potential add nolimitation to the present invention. It is easy for those skilled in theart to configure an equivalent circuit by adding modification topolarity of a transistor and each power source potential in accordancewith an art concept of the present invention, and it is clear thattheses structures are also included in the present invention.

Operation of inputting a video signal into the pixel 110 and operationof lighting and non-lighting of the pixel 110 based on the video signalis the same as the description of Embodiment Mode 1; therefore, thedescription thereof is omitted here.

Next, operation in the case where the pixel 110 is forcibly made to bein a non-lighting state is described.

When writing operation of the video signal to the pixel 110 is finished,the gate electrode of the driving transistor 106 is held at a lowpotential in a case where the light emitting layer 107 emits light andthe gate electrode of the driving transistor 106 is held at a highpotential in a case where the light emitting element 107 emits no light.

Here, the scanning line 500 is selected by a gate driver at a timing inwhich the pixel 110 is required to be forcibly in a non-lighting state,and then, the scanning line 500 is held at a high potential. Thus, thetransistor 501 is turned off. By this operation, a path of currentsupply from the power source line 101 to the light emitting element 107is blocked. Then, a potential of a first electrode of the light emittingelement 107 is immediately decreased to a potential that is about thesame as a potential of a power source connected to a terminal 108, andthe transistor 104 is turned on. Further, the transistor 105 is turnedon. Accordingly, a potential of the power source line 101 is transmittedto the gate electrode of the driving transistor 106, and the drivingtransistor 106 is turned off. Thereafter, even if the scanning line 500is held at a low potential, the driving transistor 106 has been alreadyturned off, and thus, current is not supplied to the light emittingelement 107. By the above operation, the pixel 110 that is once forciblymade to be in a non-lighting state is kept on in the non-lighting stateunless a next video signal is written.

It is to be noted that the transistor 104 may be replaced with thetransistors 600 to 603 that are connected in series (see FIG. 6). Inother words, the transistor 104 may be replaced with a transistor havinga plurality of gate electrodes that are connected to each other (alsocalled s transistor of a multi-gate structure). By using the transistorhaving a plurality of gate electrodes connected to each other, a leakagecurrent is further reduced.

Embodiment 1

A configuration of a display device of the present invention will bedescribed with reference to FIG. 7. A display device of the presentinvention includes a source driver 151, a gate driver 156, and a pixelportion 159. The source driver 151 includes a pulse output circuit 152,latch circuits 153 and 154, and a buffer circuit 155. The gate driver156 includes a pulse output circuit 157 and a buffer circuit 158. Thepulse output circuits 152 and 157 are circuits for outputting a samplingsignal, for example, a shift register and a decoder. The latch circuits153 and 154 hold a video signal and output the held video signal to acircuit in a lower stage. The buffer circuits 155 and 158 amplify theinputted signal and output the amplified signal to a circuit in thelower stage. The source driver 151 outputs a video signal to a pixel 110through a signal line. The gate driver 156 outputs a selection signal tothe pixel 110 through a scanning line.

The pixel portion 159 includes a plurality of signal lines (S1 to Sx; xis a natural number), a plurality of scanning lines (G1 to Gy; y is anatural number), a plurality of power source lines (V1 to Vx), and aplurality of pixels 110 arranged in a matrix. Each of a plurality of thepixels 110 includes a writing transistor 103, a driving transistor 106,a transistor 104, a transistor 105, and a light emitting element 107. Itis to be noted that a configuration of the pixel 110 is not limited tothe above structure, and any of the structures shown in FIGS. 3A and 3B,FIGS. 4A and 4B, FIG. 5, and FIG. 6 may be employed. Further, polarityof the transistors included in the pixel 110 is not particularlylimited.

Embodiment 2

A panel that is one mode of a display device of the present inventionwill be described with reference to FIGS. 8A and 8B. The panel includesa source driver 151 including a plurality of elements 125, a gate driver156, and a pixel portion 159 including a driving transistor 106 and alight emitting element 107 between a substrate 120 and a substrate 121(see FIGS. 8A and 8B). Further, the panel also includes a connectionfilm 122 provided over the substrate 120. The connection film 122 isconnected to a plurality of IC chips. A line A-B in FIG. 8A correspondsto a line A-B in FIG. 8B. Further, a line C-D-E in FIG. 8B correspondsto a line C-D-E in FIG. 11.

A sealing material 123 is provided around the source driver 151, thegate driver 156, and the pixel portion 159. The light emitting element107 is sealed by the sealing material 123, the substrate 120, and thesubstrate 121. A sealing process is a process for protecting the lightemitting element 107 from moisture, and here, a method of sealing with acover material (glass, ceramics, plastics, metal, or the like) is used.However, a method of sealing with a thermosetting resin or anultraviolet curable resin, or a method of sealing with a thin film thathas a high barrier ability such as metal oxide, nitride, or the like.

When a pixel electrode (a first electrode) 160 of the light emittingelement 107 has a light transmitting property and an opposed electrode(a second electrode) 161 of the light emitting element 107 has a lightshielding property, the light emitting element 107 performs bottomemission (see FIG. 8B). When the pixel electrode 160 of the lightemitting element 107 has a light shielding property and the opposedelectrode 161 of the light emitting element 107 has a light transmittingproperty, the light emitting element 107 performs top emission. Inaddition, when both the pixel electrode 160 and the opposed electrode161 of the light emitting element 107 have a light transportingproperty, the light emitting element 107 performs dual emission. Thebottom emission indicates that the light emitting element 107 emitslight toward the substrate 120. The top emission indicates that thelight emitting element 107 emits light toward the substrate 121. Thedual faces emission indicates that the light emitting element 107 emitslight toward both of the substrate 120 and the substrate 121.

The light emitting element 107 includes a layer including anelectroluminescent material (hereinafter, abbreviated as anelectroluminescent layer 162), in which luminescence generated byapplying an electric field can be obtained, between the pixel electrode160 and the opposed electrode 161. The electroluminescent layer 162 isformed by a single layer or plural layers. In these layers, an inorganiccompound may also be contained. The luminescence in theelectroluminescent layer 162 includes one or both of light emission(fluorescence) in a case of returning from a singlet excitation state toa ground state and light emission (phosphorescence) in a case ofreturning from a triplet excitation state to a ground state.

It is preferable that an element provided over the substrate 120 beformed by a thin film transistor, which has a crystalline semiconductorwith favorable characteristics such as mobility, as a channel portion.Thus, the number of external ICs that are connected can be decreased.Therefore, reduction in size, weight, and thickness of the element canbe achieved.

Further, an element provided over the substrate 120 may be formed by atransistor that has an amorphous semiconductor as a channel portion, andthe source driver 151 and the gate driver 156 may be formed by an ICchip. The IC chip is attached over the substrate 120 or is attached tothe connection film 122 by a COG method. The amorphous semiconductor canbe easily formed over a large-area substrate by using a CVD method. Inaddition, since a crystallization process is not necessary, aninexpensive panel can be provided. In this case, when a conductive layeris formed by a droplet discharging method typified by inkjet, a moreinexpensive panel can be provided.

Embodiment 3

Modes of electronic devices using a display device of the presentinvention will be described with reference to FIG. 9 and FIGS. 10A to10E. An electronic device illustrated here is a cellular phone set,which includes frame bodies 2700 and 2706, a panel 2701, a housing 2702,a printed wiring board 2703, operation buttons 2704, and a battery 2705(see FIG. 9). The panel 2701 includes a source driver 151, a gate driver156, and a pixel portion 159, and these circuits are sealed with a pairof substrates. The panel 2701 is incorporated into the housing 2702 soas to be attached and detached freely, and the housing 2702 is fittedinto the printed wiring board 2703. A shape and a size of the housing2702 are appropriately changed depending on an electronic device intowhich the panel 2701 is incorporated. A plurality of IC chips thatcorrespond to one or a plurality of circuits selected from a centralprocessing unit (CPU), a controller circuit, a power source circuit, andthe like, are mounted in the printed wiring board 2703.

The panel 2701 is connected to the printed wiring board 2703 through aconnection film 2708. A state in which the printed wiring board 2703 ismounted into the panel 2701 is called a module. The panel 2701, thehousing 2702, and the printed wiring board 2703 are stored inside theframe bodies 2700 and 2706 with the operating buttons 2704 and thebattery 2705. A pixel portion included in the panel 2701 is arranged soas to be visible from an opening window provided in the frame body 2700.

It is to be noted that the frame bodies 2700 and 2706 are shown as anexample of an outward appearance shape of the cellular phone set, and anelectronic device according to the present embodiment can be changedinto various modes in accordance with a function and usage thereof.Therefore, examples of a mode of an electronic device will be describedwith reference to FIGS. 10A to 10E below. For example, a camera such asa digital camera and a video camera, a portable game machine, a monitor,a computer, an audio reproducing device such as a car audio, an imagereproducing device provided with a recording medium such as a home gamemachine, a portable information terminal such as a PDA (see FIG. 10A), adigital video camera (see FIG. 10B), a portable television set (see FIG.10C), a notebook computer (see FIG. 10D), a television set (a televisionor a television receiver, see FIG. 10E), and the like can be given. Adisplay device of the present invention is applied to display portions700 to 705 of these electronic devices. By suppressing variation in apotential of a gate electrode of a driving transistor, a light emittingelement can be more precisely controlled to emit light or no light in adisplay device. Accordingly, an electronic device in which generation ofa display defect is suppressed can be provided by using a display deviceof the present invention.

Embodiment 4

A layout view of a pixel in a display device of the present inventionwill be described with reference to FIG. 11. The layout view of a pixel110 shown in FIG. 11 corresponds to the equivalent circuit diagram inFIG. 4B. Control of the pixel 110 is performed by a signal and apotential transmitted from each wiring of a scanning line 102, ascanning line 300, a signal line 100, and a power source line 101.Further, the pixel 110 includes a writing transistor 103, a transistor104, a transistor 105, a driving transistor 106, a transistor 400, and atransistor 112. Although it is not shown in the equivalent circuitdiagram in FIG. 4B, the pixel 110 includes a capacitor 177. One ofelectrodes of the capacitor 177 is connected to a gate electrode of thedriving transistor 106, and the other electrode is connected to thepower source line 101. In addition, in the layout view shown in FIG. 11,a pixel electrode 178 of a light emitting element 107 included in thepixel 110 is illustrated. Further, in the layout shown in FIG. 11, thewriting transistor 103 is a transistor including two gate electrodesthat are connected to each other, and the transistor 104 is a transistorincluding a plurality of gate electrodes that are connected to eachother.

This application is based on Japanese Patent Application serial no.2005-118813 filed in Japan Patent Office on April, 15, in 2005, theentire contents of which are hereby incorporated by reference.

1. A display device comprising: a pixel comprising: a first transistor;a second transistor; a pixel electrode; a circuit that controlsconduction between a gate electrode of the second transistor and a powersource line; and a controlling element that controls conduction betweenthe gate electrode of the second transistor and a second scanning line,wherein a gate electrode of the first transistor is electricallyconnected to a first scanning line, wherein one of a source electrodeand a drain electrode of the first transistor is electrically connectedto a signal line, wherein the other of the source electrode and thedrain electrode of the first transistor is electrically connected to thegate electrode of the second transistor, wherein one of a sourceelectrode and a drain electrode of the second transistor is electricallyconnected to the power source line, wherein the other of the sourceelectrode and the drain electrode of the second transistor iselectrically connected to the pixel electrode, wherein a conductionstate or a non-conduction state in the circuit is selected depending ona potential of the pixel electrode, a potential of the first scanningline, and a potential of the second scanning line, wherein thecontrolling element is to be in a conduction state or a non-conductionstate based on a potential of the second scanning line, and wherein apotential of the power source line and a potential of the secondscanning line are held to turn off the second transistor.
 2. A displaydevice according to claim 1, wherein the controlling element is a diode.3. An electronic device, wherein the display device described in claim 1is used.
 4. A display device according to claim 1, further comprising asecond electrode opposed to the pixel electrode with a light emittinglayer interposed therebetween.
 5. A display device according to claim 4,wherein the second electrode is held by a fixed potential.
 6. A displaydevice comprising: a pixel comprising: a first transistor; a secondtransistor; a pixel electrode; a third transistor; a fourth transistor;a fifth transistor; and a controlling element, wherein a gate electrodeof the first transistor is electrically connected to a first scanningline, wherein one of a source electrode and a drain electrode of thefirst transistor is electrically connected to a signal line, wherein theother of the source electrode and the drain electrode of the firsttransistor is electrically connected to a gate electrode of the secondtransistor and one of a source electrode and a drain electrode of thethird transistor, wherein one of a source electrode and a drainelectrode of the second transistor is electrically connected to a powersource line, wherein the other of the source electrode and the drainelectrode of the second transistor is electrically connected to thepixel electrode, a gate electrode of the third transistor, and one ofterminals of the controlling element, wherein the other of the sourceelectrode and the drain electrode of the third transistor iselectrically connected to one of a source electrode and a drainelectrode of the fourth transistor, wherein a gate electrode of thefourth transistor is electrically connected to the first scanning line,wherein the other of the source electrode and the drain electrode of thefourth transistor is electrically connected to one of a source electrodeand a drain electrode of the fifth transistor, wherein the other of thesource electrode and the drain electrode of the fifth transistor iselectrically connected to the power source line, wherein the otherterminal in the controlling element is connected to a second scanningline, and wherein a potential of the power source line and a potentialof the second scanning line are held to turn off the second transistor.7. A display device according to claim 6, wherein the controllingelement is a diode.
 8. A display device according to claim 6, whereinthe controlling element is a sixth transistor, wherein one of terminalsof the controlling element is a gate electrode and a drain electrode ofthe sixth transistor, and wherein the other terminal of the controllingelement is a source electrode of the sixth transistor.
 9. A displaydevice according to claim 6, wherein the third transistor is atransistor having a plurality of gate electrodes that are connected toeach other.
 10. A display device according to claim 6, wherein polarityof the first transistor and polarity of the fourth transistor aredifferent from each other.
 11. An electronic device, wherein the displaydevice described in claim 6 is used.
 12. A display device according toclaim 6, further comprising a second electrode opposed to the pixelelectrode with a light emitting layer interposed therebetween.
 13. Adisplay device according to claim 12, wherein the second electrode isheld by a fixed potential.